High-quality torque regulation for polyphase AC motors depends upon accurate control of instantaneous current in each of the motor phase windings. Such control requires that instantaneous phase current levels be accurately sensed. Present current-controlled AC drives use discrete current sensors in series with the motor phase windings to sense these phase current levels. One prior art technique of sensing current flow through such phase winding involves sensing the voltage drop developed across a resistor that is serially connected to the winding. Since the possible current range is large, the sensing resistor must have a relatively high power rating, which increases its cost. Alternatively, the current sensors may constitute high-quality Hall-effect transducers which are also relatively expensive. All these current sensors must be galvanically isolated from each other, as well as from the control logic, and must reject significant common-mode signals associated with the inverter switching in order to maintain integrity of the current data. While Hall-effect current sensors can meet these isolation requirements, they are temperature sensitive, bulky and fragile, and their output signals are subject to offset and drift. High-quality Hall-effect devices with improved performance characteristics are available, but at very high cost.
In addition to the above disadvantages, the volume and isolation requirements for discrete current sensors limit the possible reduction in size and weight of AC drives to be obtained through their use, even though the physical size of power semiconductors used in inverters continues to shrink. A power semiconductor device, such as an insulated gate bipolar transistor (IGBT) having a main current section and a current sensing section that emulates or follows the current in the main current section, is disclosed in Walden and Wildi patent application Ser. No. 892,739, filed July 31, 1986 as a file wrapper continuation of Ser. No. 529,240, filed Sept. 6, 1983, and assigned to the assignee of the present application. Although some IGBT switches having integrated current sensors have been fabricated into some AC drives, they function in those circuits solely to provide overcurrent threshold detection. As such, they do not provide the complete current information necessary for closed-loop current regulation.